Optical sensors have previously been proposed to sense the course of a combustion event occurring within the combustion chamber of an IC engine, to determine therefrom the course of pressure variation arising upon combustion of an air-fuel mixture. Some such sensors utilize a housing similar to a spark plug, and which may be combined with or formed part of a spark plug, for insertion into the engine block of the IC engine, through a suitable tapped opening. The referenced application No. 214,481, the disclosure of which is hereby incorporated by reference, describes a type of such a sensor.
Combining an optical sensor with a spark plug has substantial advantages, since no special openings need be formed in the cylinder head of the IC engine, and standard arrangements may therefore be used. It is only necessary to provide a dual connection to the so-constructed plug, one for the electrical supply to transmit ignition to transmit ignition energy to the spark plug portion of the combination and one to connect an optical light guide to provide for optical sensing of combustion phenomena arising in the cylinder of the IC engine. In order to conduct electrical energy to the spark gap in such combined sensors, it has been proposed to surround a glass rod, forming the optical pickup, typically a quartz glass rod, with a metalicsleeve, surrounding the glass rod. In the arrangements, the glass rod was threaded through the opening of the metalic sleeve.
It has been found that this arrangement is expensive to manufacture since the metalic sleeve is very long with respect to the diameter of the opening therein, resulting in difficulties of insertion of the glass rod. In mass production conditions, during insertion of a glass rod into the opening of a sleeve having a large length-to-diameter ratio, problems with centering of the opening in the metal sleeve or tube arise. If the opening within the sleeve, which, essentially, is a long tube, is not perfectly aligned and centered, the glass rod may break. Additionally, it is difficult to secure a thin glass rod within a small opening in such a metal tube or sleeve so that it is securely held therein while, also, being seated and sealed therein so that combustion gasses, under the high pressures which arise during explosion and combustion of the fuel-air mixture will not leak out from the side of the glass tube.
The referenced German Patent Disclosure Document describes a spark plug which includes a resistor made of a heterogeneous mixture of conductive materials, so that the spark plug, inherently, is a "resistor" type plug forming its own radio noise suppression element.